Importing Photoshop Brushes for Grease Pencil and Texture Mapping?
There is a way around this. You can convert photoshop custom brushes (ABR files) into PNG files using the free OpenSource programme ABRViewer. Alternatively just save a masked PNG that you want to use as a brush.

Using Grease Pencil in Blender 2.8 make sure you're in Draw mode. Then click on the Active tool tab. Change brush name and make other changes to settings you might want.



Then add a new material (in material tab). Under the checked Stroke section change Style to Texture.

 

Under this will appear an Open box. Click this to browse for the brush PNG you want to use.



I change Mode Type to Dots.
If you want to use your own colour, rather than the colour of the brush, check Use As Stencil Mask and then change Color.



Perhaps these tutorials can help:

How to Use Photoshop Brushes in Other Programs (specifically exporting them as PNG files)

Grease Pencil Textured Brushes - Blender 2.80 Beta

Creating your own custom Grease Pencil brushes (Blender 2.8) (using PNG files with mask)